The preceding discussion explained how to measure a true course
on the aeronautical chart and how to make corrections for variation and
deviation, but one important factor has not been considered—wind. As discussed
in the study of the atmosphere, wind is a mass of air moving over the surface
of the Earth in a definite direction. When the wind is blowing from the
north at 25 knots, it simply means that air is moving southward over the
Earth’s surface at the rate of 25 NM in 1 hour.
Under these conditions, any inert object free from contact with
the Earth will be carried 25 NM southward in 1 hour. This effect becomes
apparent when clouds, dust, toy balloons, etc., are observed being blown
along by the wind. Obviously, an airplane flying within the moving mass
of air will be similarly affected. Even though the airplane does not float
freely with the wind, it moves through the air at the same time the air
is moving over the ground, thus is affected by wind. Consequently, at the
end of 1 hour of flight, the airplane will be in a position which results
from a combination of these two motions:

• the movement of the air mass in reference to the ground, and
• the forward movement of the airplane through the air mass.

Actually, these two motions are independent. So far as the airplane’s
flight through the air is concerned, it makes no difference whether the
mass of air though which the airplane is flying is moving or is stationary.
A pilot flying in a 70-knot gale would be totally unaware of any wind (except
for possible turbulence) unless the ground were observed. In reference
to the ground, however, the airplane would appear to fly faster with a
tailwind or slower with a headwind, or to drift right or left with a crosswind.

As shown in figure 8-12, an airplane flying eastward at an airspeed
of 120 knots in calm wind, will have a groundspeed exactly the same—120
knots. If the mass of air is moving eastward at 20 knots, the airspeed
of the airplane will not be affected, but the progress of the airplane
over the ground will be 120 plus 20, or a groundspeed of 140 knots. On
the other hand, if the mass of air is moving westward at 20 knots, the
airspeed of the airplane still remains the same, but groundspeed becomes
120 minus 20 or 100 knots.

Assuming no correction is made for wind effect, if the airplane is
heading eastward at 120 knots, and the air mass moving southward at 20
knots, the airplane at the end of 1 hour will be 120 miles east of its
point of departure because of its progress through the air. It will be
20 miles south because of the motion of the air. Under these circumstances,
the airspeed remains 120 knots, but the groundspeed is determined by combining
the movement of the airplane with that of the air mass.Groundspeed can
be measured as the distance from the point of departure to the position
of the airplane at the end of 1 hour.

Figure 8-12.—Motion of the air affects the speed with which airplanes
move over the Earth's surface. Airspeed, the rate at which an airplane
moves through the air, is not affected by air motion.

The groundspeed can be computed by the time required to
fly between two points a known distance apart. It also can be determined
before flight by constructing a wind triangle, which will be explained
later in this chapter. [Figure 8-13]

Figure 8-13.—Airplane flightpath resulting from its airspeed and
direction, and the windspeed and direction.

The direction in which the plane is pointing as it flies is heading.
Its actual path over the ground, which is a combination of the motion of
the airplane and the motion of the air, is track. The angle between the
heading and the track is drift angle. If the airplane’s heading coincides
with the true course and the wind is blowing from the left, the track will
not coincide with the true course. The wind will drift the airplane to
the right, so the track will fall to the right of the desired course or
true course. [Figure 8-14]

Figure 8-14.—Effects of wind drift on maintaining desired course.

By determining the amount of drift, the pilot can counteract
the effect of the wind and make the track of the airplane coincide with
the desired course. If the mass of air is moving across the course from
the left, the airplane will drift to the right, and a correction must be
made by heading the airplane sufficiently to the left to offset this drift.
To state in another way, if the wind is from the left, the correction will
be made by pointing the airplane to the left a certain number of degrees,
therefore correcting for wind drift. This is wind correction angle and
is expressed in terms of degrees right or left of the true course. [Figure
8-15]

Figure 8-15.—Establishing a wind correction angle that will counteract
wind drift and maintain the desired course.

To summarize:

• COURSE— is the intended path of an aircraft over the Earth; or the
direction of a line drawn on a chart representing the intended aircraft
path, expressed as the angle measured from a specific reference datum clockwise
from 0° through 360° to the line.
• HEADING— is the direction in which the nose of the airplane points
during flight.
• TRACK—is the actual path made over the ground in flight. (If proper
correction has been made for the wind, track and course will be identical.)
• DRIFT ANGLE—is the angle between heading and track.
• WIND CORRECTION ANGLE—is correction applied to the course to establish
a heading so that track will coincide with course.
• AIRSPEED—is the rate of the airplane’s progress through the air.
• GROUNDSPEED—is the rate of the airplane’s in-flight progress over
the ground.